Is there some kind of temp sensor for a battery pack for 20 battery cell, I'm thinking of a flat surface that will act as the temp sensor for the whole battery pack plss help for undergrad thesis or any other advice or option.

 

Any Ntc thermistor will do, try a 10K type..

 

With a 20-cell pack you may need more than one sensor, placed near the pack centre to detect the hottest cells.

 

You could also consider using something like one of these (KSD-9700):
https://www.aliexpress.com/af/KSD%252d9700.html?d=y&origin=n&SearchText=KSD-9700&catId=0&initiative_id=SB_20191214092832
- A thermostat if you dont want the added complication of measuring the NTC voltage.